Ethernet.ppt

Ethernet
Dr.T.Thendral, Assistant Professor, SRCW

• Ethernet Cabling
• Manchester Encoding
• The Ethernet MAC Sublayer Protocol
• The Binary Exponential Backoff Algorithm
• Ethernet Performance
• Switched Ethernet
• Fast Ethernet
• Gigabit Ethernet
• IEEE 802.2: Logical Link Control
• Retrospective on Ethernet

Ethernet Cabling
• Finished our general discussion of channel allocation
protocols
• It is time to see how these principles apply to real
systems, in particular, LANs
• IEEE has standardized a number of local area networks
and metropolitan area networks under the name of
IEEE 802
• The most important of the survivors are 802.3
(Ethernet) and 802.11 (wireless LAN)
• With 802.15 (Bluetooth) and 802.16 (wireless MAN)
• Ethernet, the protocols and recent developments in
highspeed (gigabit) Ethernet

Ethernet Cabling
• ''Ethernet'' refers to the cable (the ether)
• Four types of cabling are commonly used, as
shown in Fig

• Detecting cable breaks, excessive length, bad taps, or loose
connectors can be a major problem with this media
• For this reason, techniques have been developed to track them
down
• Basically, a pulse of known shape is injected into the cable
• If the pulse hits an obstacle or the end of the cable, an echo will be
generated and sent back
• By carefully timing the interval between sending the pulse and
receiving the echo, it is possible to localize the origin of the echo
• This technique is called time domain reflectometry
• The problems associated with finding cable breaks drove systems
toward a different kind of wiring pattern, in which all stations have
a cable running to a central hub in which they are all connected
electrically

• Three wiring schemes are illustrated in Fig

• For 10Base5, a transceiver is clamped securely
around the cable so that its tap makes contact
with the inner core
• The transceiver contains the electronics that
handle carrier detection and collision detection
• When a collision is detected, the transceiver also
puts a special invalid signal on the cable to ensure
that all other transceivers also realize that a
collision has occurred

• 10Base5: a transceiver cable or drop cable
connects the transceiver to an interface board in
the computer
• The interface board contains a controller chip
that transmits frames to, and receives frames
from, the transceiver.
• 10Base2: The connection to the cable is just a
passive BNC T-junction connector
• The transceiver electronics are on the controller
board, and each station always has its own
transceiver.

• 10Base-T: There is no shared cable at all, just the
hub (a box full of electronics) to which each
station is connected by a dedicated (i.e., not
shared) cable
• A fourth cabling option for Ethernet is 10Base-F,
which uses fiber optics
• This alternative is expensive due to the cost of
the connectors and terminators, but it has
excellent noise immunity and is the method of
choice when running between buildings

• A repeater is a physical layer device
• It receives, amplifies (regenerates), and retransmits signals in both
directions
• As far as the software is concerned, a series of cable segments
connected by repeaters is no different from a single cable (except
for some delay introduced by the repeaters)

Manchester Encoding
• None of the versions of Ethernet uses straight binary encoding
with 0 volts for a 0 bit and 5 volts for a 1 bit because it leads to
ambiguities
• The start, end, or middle of each bit without reference to an
external clock
• Two such approaches are called Manchester encoding and
differential Manchester encoding
• With Manchester encoding, each bit period is divided into two
equal intervals
• A binary 1 bit is sent by having the voltage set high during
the first interval and low in the second one
• A binary 0 is just the reverse: first low and then high

Manchester encoding is shown in Fig. 4-16(b)
Differential Manchester encoding, shown in Fig. 4-16(c), is a variation of basic
Manchester encoding

The Ethernet MAC Sublayer Protocol
• The original DIX (DEC, Intel, Xerox) frame
structure is shown in Fig
• Each frame starts with a Preamble of 8 bytes
• The frame contains two addresses, one for the
destination and one for the source
• When a frame is sent to a group address, all the
stations in the group receive it
• Sending to a group of stations is called multicast
• The address consisting of all 1 bits is reserved for
broadcast

• Station tries to transmit a very short frame, it is conceivable that
a collision occurs, but the transmission completes before the
noise burst gets back at 2τ
• The sender will then incorrectly conclude that the frame was
successfully sent
• To prevent this situation from occurring, all frames must take
more than 2τ to send so that the transmission is still taking place
when the noise burst gets back to the sender.

• The final Ethernet field is the Checksum
• It is effectively a 32-bit hash code of the data
• If some data bits are erroneously received (due to
noise on the cable), the checksum will almost
certainly be wrong and the error will be detected
• The checksum algorithm is a cyclic redundancy check
(CRC)
• It just does error detection, not forward error
correction

The Binary Exponential Backoff Algorithm
• In general, after i collisions, a random number between 0 and
2i
• - 1 is chosen, and that number of slots is skipped
• However, after ten collisions have been reached, the
randomization interval is frozen at a maximum of 1023 slots
• After 16 collisions, the controller throws in the towel and
reports failure back to the computer
• Further recovery is up to higher layers
• This algorithm, called binary exponential backoff

Ethernet Performance
• The performance of Ethernet under conditions of heavy and
constant load, that is, k stations always ready to transmit
• A rigorous analysis of the binary exponential backoff
algorithm is complicated
• Instead, we will follow Metcalfe and Boggs (1976) and
assume a constant retransmission probability in each slot
• The channel efficiency is plotted versus number of ready
stations

Switched Ethernet
• The heart of this system is a switch containing a high-
speed backplane and room for typically 4 to 32 plug-
in line cards, each containing one to eight connectors
• Most often, each connector has a 10Base-T twisted
pair connection to a single host computer

Figure 4-20. A simple example of switched Ethernet.

Fast Ethernet
• To pump up the speed, various industry groups
proposed two new ring-based optical LANs
• One was called FDDI (Fiber Distributed Data
Interface) and the other was called Fibre Channel
• In any event, the failure of the optical LANs to
catch fire left a gap for garden-variety
• Ethernet at speeds above 10 Mbps
• Many installations needed more bandwidth and
thus had numerous 10-Mbps LANs connected by
a maze of repeaters, bridges, routers, and
gateways

• The 802.3 committee decided to go with a
souped-up Ethernet for three primary reasons:
• 1. The need to be backward compatible with
existing Ethernet LANs.
• 2. The fear that a new protocol might have
unforeseen problems.
• 3. The desire to get the job done before the
technology changed

Gigabit Ethernet
• Fast Ethernet standard when the 802
committee began working on a yet faster
Ethernet (1995)
• It was quickly dubbed gigabit Ethernet and
was ratified by IEEE in 1998 under the name
802.3z

• In the simplest gigabit Ethernet configuration, illustrated in
Fig. 4-22(a), two computers are directly connected to each
other
• common case, having a switch or a hub connected to multiple
computers
• and possibly additional switches or hubs, as shown in Fig. 4-
22(b)
• In both configurations each individual Ethernet cable has
exactly two devices on it, no more and no fewer

• Gigabit Ethernet uses new encoding rules on the
fibers
• The following two rules were used in making the
choices:
• 1. No codeword may have more than four identical
bits in a row.
• 2. No codeword may have more than six 0s or six 1s.

• There are also systems in which an error-
controlled, flow-controlled data link protocol
is desired
• IEEE has defined one that can run on top of
Ethernet and the other 802 protocols
• In addition, this protocol, called LLC (Logical
Link Control)

• LLC forms the upper half of the data link layer, with the
MAC sublayer below it, as shown in Fig
• Typical usage of LLC is as follows
• The network layer on the sending machine passes a packet to
LLC, using the LLC access primitives
• The LLC sublayer then adds an LLC header, containing
sequence and acknowledgement numbers

Retrospective on Ethernet
• Ethernet has been around for over 20 years and has
no serious competitors in sight, so it is likely to be
around for many years to come
• Few CPU architectures, operating systems, or
programming languages have been king of the
mountain for two decades going on three
• the main reason for it that Ethernet is simple and
flexible
• Ethernet has been able to evolve in certain crucial
ways

Ethernet.ppt

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